This invention relates to a transmission line switching system for use in a duplex transmission network comprising a duplex transmission line.
A duplex transmission network of the type described comprises first and second transmission lines as the duplex transmission line and first and second transmitting devices connected to the first and the second transmission lines, respectively. Each of the first and second transmitting devices receives a virtual container signal (VC) which contains digital data as known in the art. The first and second transmitting devices transmit, to the first and second transmission lines, first and second transmission line frame signals, respectively, each of which comprises an original pointer and the virtual container in each transmission line frame. The original pointer designates the location of a leading edge of the virtual container in each transmission line frame.
Each of the first and the second transmission line frame signals is typically a synchronous transfer mode (STM) signal comprising a section overhead (SOH) which precedes the virtual container. The original pointer is included in the section overhead.
The duplex transmission network further comprises first and second receiving devices comprising first and second pointer renewing units connected to the first and second transmission line, respectively. The first and second pointer renewing units receive the first and second transmission line frame signals from the first and second transmission lines as first and second incoming frame signals, respectively. Between the first and second incoming frame signals, a phase difference is inevitably present due to a difference in length between the first and second transmission lines. The first and second pointer renewing units produce first and second device frame signals, respectively, each of which comprises a renewed pointer and the virtual container in each device frame. The renewed pointer designates the renewed location of the leading edge of the virtual container in each device frame. The device frame of the second device frame signal is synchronized with the device frame of the first device frame signal. In other words, a phase difference is not present between the device frame of the first device frame signal and the device frame of the second device frame signal.
Connected to the first and second pointer renewing units, a selecting unit selects one of the first and second device frame signals as a selected device frame signal in response to a selection signal to make a subsequent unit extract the virtual container from the selected device frame signal on the basis of the renewed pointer of each device frame. The subsequent unit is typically a synchronous digital hierarchy (SDH) terminator known in the art.
As will later be described, a conventional transmission line switching system comprises, in each of the first and second pointer renewing units, an elastic buffer memory, a write controller for controlling writing of the virtual container into the buffer memory, a read controller for controlling reading of the virtual container from the buffer memory, and a phase comparing circuit for comparing a reading phase of the read controller with a writing phase of the write controller to produce a primary stuff request signal and deliver the primary stuff request signal to the read controller. The conventional transmission line switching system is incapable of switching without interruption between the first and the second device frame signals. This is because the leading edge of the virtual container of each device frame of the second device frame signal is not in synchronism with the leading edge of the virtual container of each device frame of the first device frame signal. That is, a phase difference is present between the virtual containers of the first and second device frame signals.